Method for Evaluating and Modifying Solder Attach Design for Integrated Circuit Packaging Assembly

ABSTRACT

A method of reducing a likelihood that a die pad will be delaminated from a die in an integrated circuit die package for a structure design during an attachment of a heat sink member to the die pad using solder, is provided. A sample structure of the structure design is evaluated to determine whether a volume of last solidification for the solder is centrally located with respect to the die pad and is located at or near an interface of the solder and the die pad. If the last solidification volume is centrally located and is located at or near the interface of the solder and the die pad, and if the die pad is delaminated from the die, the structure design is modified so that less metal of the heat sink member is centrally located than before the modifying.

TECHNICAL FIELD

The present invention relates generally to integrated circuit packagingand assembly, and testing thereof. In one aspect it relates moreparticularly to evaluating and modifying a solder attach structuredesign for an integrated circuit packaging assembly.

BACKGROUND

Heat dissipation from an integrated circuit (IC) chip or die duringoperation is typically an important issue, especially as the density ofIC devices on a die continues to increase. Also, many devices now havecombinations of high-power transistors and low-power transistors formedon a same die. Such high-power transistors tend to produce more heatthan low-power transistors. Further, more system-on-chip configurationsare being used. Thus, there are often a wide variety of IC devices on asame die. Some of the IC devices can handle and/or put out much moreheat than nearby or neighboring devices on the same die. Hence, thereliability and effectiveness of heat dissipation for a packaged IC diemay greatly affect the reliability and/or performance of an IC chipduring operation.

Many die packages 22 have an exposed die pad surface 24, as shown inFIG. 1 for example. Typically a die 26 is attached to or bonded to a diepad 28 to improve heat transfer from the die 26 via the die pad 28. Anexposed die pad 28 is often soldered to a heat sink member 30, as shownin FIG. 2 for example, or other metal components on a printed circuitboard (PCB) 34 to provide a primary heat transfer path from the die 26to the heat sink 30 via the die pad 28. The heat sink 30 in FIG. 2 is ametal rivet that extends through the PCB 34. Many die pads 28 of diepackages 22 are thin to help reduce package size (e.g., packagethickness). As a result, many die pads 28 are flexible and easilydeformed (like a diaphragm). In such cases, it has been found that theforces exerted on a die pad 28 by solidifying solder 36 may be greatenough to cause delamination between the die pad 28 and the die 26 wherethe die 26 is supposed to be attached to the die pad 28. Suchdelamination may greatly reduce the heat transfer efficiency and hinderthe thermal path for cooling the die 26 via the die pad 28. It would bepreferred to reduce the probability that a die pad 28 may be delaminatedfrom a die 26 or deformed by a solder attachment procedure due to soldersolidification forces.

SUMMARY OF THE INVENTION

The problems and needs outlined above may be addressed by embodiments ofthe present invention. In accordance with one aspect of the presentinvention, a method of reducing a likelihood that a die pad will bedelaminated from a die in an integrated circuit die package for astructure design during an attachment of a heat sink member to the diepad using solder, is provided. This method assumes that the heat sinkmember includes metal. A sample structure of the structure design isevaluated to determine whether a volume of last solidification for thesolder is centrally located with respect to the die pad and is locatedat or near an interface of the solder and the die pad. If the lastsolidification volume is centrally located and is located at or near theinterface of the solder and the die pad, and if the die pad isdelaminated from the die, the structure design is modified so that lessmetal of the heat sink member is centrally located than before themodifying.

In accordance with another aspect of the present invention, a method ofreducing a likelihood that a die pad will be delaminated from a die inan integrated circuit die package for a structure design during anattachment of a heat sink member to the die pad using solder, isprovided. A sample structure of the structure design is evaluated todetermine whether a volume of last solidification for the solder is ator near an interface of the solder and the die pad and is centrallylocated with respect to the die pad. If the last solidification volumeis at or near the interface of the solder and the die pad, the structuredesign is modified so that the last solidification volume is locatedfarther from the interface of the solder and the die pad than before themodifying and/or is located farther from a central location of the diepad in a direction along a plane of the die pad than before themodifying.

In accordance with yet another aspect of the present invention, a methodof reducing a likelihood that a die pad will be delaminated from a diein an integrated circuit die package during an attachment of at leastone heat sink member to the die pad using solder, is provided. Astructure having a minimum solder thickness of greater than about 4 milsbetween the die pad and the at least one heat sink member is evaluatedto determine whether a volume of last solidification for the solder isat or near an interface of the solder and the die pad. If the lastsolidification volume is at or near the interface of the solder and thedie pad, the design of the at least one heat sink member is modified sothat the last solidification volume will be shift farther away from theinterface of the solder and the die pad than before the modifying.

The foregoing has outlined rather broadly features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention. It should be appreciated by those skilledin the art that the conception and specific embodiment disclosed may bereadily utilized as a basis for modifying or designing other structuresor processes for carrying out the same purposes of the presentinvention. It should also be realized by those skilled in the art thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which illustrateexemplary embodiments of the present invention and in which:

FIG. 1 is a cross-section view of a typical die package with an exposeddie pad;

FIG. 2 is a cross-section view showing the die package of FIG. 1attached to a heat sink member with solder and attached to a printedcircuit board;

FIGS. 3A-3C and 4 show detailed cross-section views of two illustrativeexamples under evaluation;

FIG. 5 is simplified schematic modeling the negative pressure exerted onthe die pad due to solder shrinkage;

FIG. 6 shows a cross-section schematic of the test apparatus;

FIG. 7 is surface map of a bowed die pad from a Moire experiment;

FIG. 8 is surface map of a bowed die pad from another Moire experiment;

FIG. 9 is a simplified cross-section view of the structure of FIG. 2illustrating a last solidification volume evidenced by one or morevoids;

FIG. 10 is a top view of solidified solder on the rivet with the die padremoved;

FIGS. 11-13 illustrate a modified heat sink rivet in a structure thatshifts the last solidification volume farther away from the interface ofthe solder and the die pad;

FIG. 14 is a perspective view showing a part of the structure cut-awayfrom a printed circuit board;

FIG. 15 is a perspective view of the printed circuit board with a heatsink member (nine metal vias) formed therein;

FIG. 16 is a top view of the solder on the heat sink members (design ofFIG. 15) as taken along line A-A in FIG. 14;

FIG. 17 is cross-section view of the structure of FIGS. 15 and 16 astaken along line B-B in FIG. 14;

FIG. 18 is a perspective view of the printed circuit board with amodified heat sink member (eight metal vias) formed therein;

FIG. 19 is a top view of the solder on the heat sink members (design ofFIG. 18) as taken along line A-A in FIG. 14; and

FIG. 20 is cross-section view of the structure of FIGS. 18 and 19 astaken along line B-B in FIG. 14.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to the drawings, wherein like reference numbers are usedherein to designate like or similar elements throughout the variousviews, illustrative embodiments of the present invention are shown anddescribed. The figures are not necessarily drawn to scale, and in someinstances the drawings have been exaggerated and/or simplified in placesfor illustrative purposes only. One of ordinary skill in the art willappreciate the many possible applications and variations of the presentinvention based on the following illustrative embodiments of the presentinvention.

FIG. 1 is a cross-section view of a typical die package 22 with anexposed die pad 28. The die 26 is attached to the die pad 28 with a diemount compound 38 (e.g., adhesive) to provide efficient heat transferfrom the die 26 out of the package 22 via the die pad 28. FIG. 2 is across-section view showing the die package 22 of FIG. 1 attached to aheat sink member 30 with solder 36 and attached to a PCB 34.

Generally, an embodiment of the present invention provides a method ofreducing the likelihood that a die pad 28 will be delaminated from a die26 during an attachment of a heat sink member 30 to the die pad 28 usingsolder 36. In one embodiment, this method includes a step of evaluatinga structure 40 to determine whether a volume of last solidification forthe solder 36 attaching the die pad 28 to the heat sink member 30 is ator near an interface 42 of the solder 36 and the die pad 28. An exampleof such evaluation will be described in more detail below. This methodalso includes a step of modifying the design of the structure 40 or theheat sink member 30 so that the last solidification volume for thesolder 36 will be shifted farther away (vertically) from the diepad-solder interface 42 and/or farther (along PCB plane) from a centralregion of the die pad 28. Illustrative examples of modifying heat sinkmember designs are shown and discussed below.

Evaluations of production structures 40 having essentially the samedesign shown in FIG. 2 were performed to investigate the cause of thedie pad 28 becoming delaminated from the die 26. FIGS. 3A-3C and 4 showdetailed cross-section views of two illustrative examples. FIGS. 3A-3Cshow a same sample of a structure 40 in three views. FIG. 3A shows aportion of the structure 40 at a left end 45 of the die 26, FIG. 3Bshows a central portion of the structure 40, and FIG. 3C shows a portionof the structure 40 at a right end 46 of the die 26 (along the die padplane). FIG. 4 shows a central portion of the same rivet structure 40for another sample.

Briefly, the parts of the structure 40 shown in FIGS. 3A-4 will bedescribed to provide a context for the discussion of these figures.Reference also may be made to FIG. 2 to understand the context of FIGS.3A-4. A die 26, having integrated circuit devices 48 formed thereon, isattached to a die pad 28 by a die mount compound 38 (e.g., silver filledadhesive) to provide an intended heat transfer path between the die 26and the die pad 28. The die 26 is covered by package mold compound 50(e.g., plastic) to protect the die 26. A left end 45 of the die 26 isshown in FIG. 3A. A right end 46 of the die 26 is shown in FIG. 3C. ThePCB 34 is shown in FIGS. 3A and 3C. The heat sink rivet 30 shown inFIGS. 3A-3C is extending through a hole 52 in the PCB 34 (refer also toFIG. 2 for context). In FIGS. 3A and 3C, copper traces 54 are shown onthe top surface of the PCB 34, to which the solder is also attached. Thesolder 36 is located between the heat sink rivet 30 and the die pad 28.FIG. 4 is the same view as FIG. 3B, but for a different sample of thesame structure design.

As shown in FIG. 3B, a void 56 is formed between the die pad 28 and thedie 26 due to delamination. A close study of this sample revealed thatthe die pad 28 was pulled away from the die 26 and bowed downward towardthe heat sink member 30 at the central portion (see FIG. 3B). A study ofthis sample also revealed air pockets (bubbles) 58 at the interface 42of the solder 36 and the die pad 28 in the central portion (see FIG.3B). The air pockets 58 shown in FIG. 3B indicate that this region 58 isthe last solidification volume for the solder 36 during the cooling.FIG. 4 shows one large air pocket region 58 at a central portion of thestructure 40. In FIG. 4, a void 56 is also formed between the die pad 28and the die 26 due to delamination. FIGS. 3A and 3B reveal end portionsof the void 56 shown in FIG. 3B.

Referring to FIG. 3B, the solder thickness in this sample structure 40was about 7.4 mils at the central portion of the structure 40 and thevoid 56 had a thickness of about 1.3 mils at the central portion. InFIGS. 3A and 3C, the solder thickness at the PCB 34 is about 3.7 milsand the solder thickness at the edges of the rivet 30 is about 13.4mils. In FIG. 4, the solder thickness is about 7.9 mils and the void 56has a thickness of about 0.5 mils at the central portion of thestructure 40.

There are a number of reasons that contribute to the delaminationproblem. During the heating of the structure 40 to reflow the solder 36(while attaching the die package 22 to the heat sink member 30 and thePCB 34), the die mount compound 38 becomes softer and its adhesionstrength is decreased. This allows the die pad 28 to be more easilybowed like a diaphragm when not sufficiently supported (e.g., supportedby the die mount compound 38). Thus, it does not take much downwardpressure on the die pad 28 to create a delamination void 56 between thedie 26 and the die pad 28 during the reflow process. Also, the heat sinkrivet 30 has a much greater mass than the die pad 28. The greater massof the heat sink rivet 30 causes it to cool much slower than the die pad28. The heat sink rivet 30 is also typically in contact with or attachedto an even larger aluminum plate or fin (not shown). Furthermore, theheat sink rivet 30 of the structure design shown in FIGS. 2-4 is madefrom solid metal (as opposed to being hollow). As a result, the heatsink rivet 30 tends to cool from the outside toward its center. Anotherpoint to note is that the heat sink rivet 30 of this design ismechanically attached to the PCB 34 and its position relative to the topsurface of the PCB 34 is often inconsistent during production. As shownin FIGS. 3A-3C, the top of the heat sink rivet 30 is not level with thetop of the PCB 34 in this sample. This causes the solder 36 to bethicker at the heat sink rivet 30 than at the PCB 34. It is preferred tohave a minimum solder thickness between the die pad 28 and the heat sinkrivet 30 of about 4 mils or less. However, as shown in FIGS. 3B and 4,this minimum solder thickness is sometimes much greater than 4 mils(e.g., 7.4 mils in FIG. 3B, 7.9 mils in FIG. 4). As the solder 36 cools(from liquid phase to solid phase), its volume decreases by about 4%(for typical solder). Because the heat sink rivet 30 cools from theoutside inward, a volume of last solidification for the solder tends toget trapped and concentrated in the central portion. The shrinkage ofthe cooling solder 36 at this last solidification volume 58, which istrapped between the already solidified solder (around the outside), thedie pad 28, and the heat sink rivet 30, is then concentrated at thecentral portion. This trapped solder shrinkage causes a negative ordownward suction pressure on the die pad 28, which pulls the centralportion of the die pad 28 away from the die 26. These factors andconditions contribute to causing the die pad 28 to be delaminated fromthe die 26 during assembly. Thus, by eliminating or taking into accountthese factors, the likelihood that a die pad 28 will be delaminated froma die 26 during assembly may be reduced or even significantly reduced.This may be done by modifying the design of the heat sink member 30, forexample, as described further below.

FIG. 5 is simplified schematic modeling the negative pressure exerted onthe die pad 28 due to solder shrinkage during cooling at a trappedcentral portion of the structure 40. In FIG. 5, a die pad 28, solder 36,and a heat sink member 30 are shown. Using this structure model of FIG.5, bending moment beam and displacement equations may be used toestimate the pressure required to cause delamination. The followingbasic equations may be used:Y _(max)=5Wl ⁴/384EI,I=bh ³/12,σ=Mc/I, andM=Wl ²/8,

where Y_(max) is the maximum allowable deflection (see FIG. 5) withoutcausing delamination, l is the length of the trapped liquid solder 36(see FIG. 5), W is an equally distributed pressure from the solder 36over the length l, E is the Young's modulus for the die pad material(e.g., in pounds/inch²), I is the moment of inertia of the die pad beamcross-section (e.g., in inches⁴), b is the depth (Z axis) of the beamcross-section (e.g., in inches), h is the thickness of the die pad 28(see FIG. 5), σ is the stress in the leadframe material at the distancec (e.g., in pounds/inch²) on the surface of the leadframe, M is momentabout the ends of the die pad length l, and c is the maximum distancefrom the neutral axis consisting of half of the leadframe thickness(e.g., in inches).

A calculation using the model of FIG. 5 and these equations in onecalculation shows that a pressure W of about 1.92 psi will likelydeflect the die pad enough to cause delamination, i.e., based upon thefollowing assumptions: Y_(max) is about 0.001 inch (about 1 mil), lengthl is about 0.287 inch, h is about 0.005 inch, the die pad is made fromCDA-194 leadframe alloy, E is 17×10⁶ psi, and the yield stress of theleadframe material is about 70×10³ psi. In another calculation, apressure W of about 10.6 psi will likely deflect the die pad by about0.0013 inch, while using the same assumptions as the prior calculation,except that the length l is shortened to be 0.200 inch. These arerelatively low stress levels. These calculations reveal that the soldershrinkage may be a valid mechanism for causing delamination.

It is also noted that the material stress on the leadframe typicallywill be about 13 k psi, which is about 19% of the yield stress for theleadframe material. This is the stress that is exerted on the leadframematerial, resulting from liquid solder solidifying. The liquid solderwill transfer the force of the shrinking solder to the leadframe (like anegative pressure). Because the leadframe material has not been stressedbeyond the yield point, a void 56 between the die 26 and the die pad 28in an unsoldered package 22 (removed from the PCB 34 and heat sink rivet30) will likely be closed up by this material stress, which may makedetection of such voids difficult.

A test apparatus 60 (see FIG. 6) was built to trap the volume of lastsolidification for solidifying solder 36 against a die pad blank 62 at acentral location and to measure the curvature of the die pad blank 62from the negative pressure of the solidifying solder 36. Standard beamequations (as discussed above regarding FIG. 5) were used to estimatethe pressure needed to cause the curvature under the specific testarrangement. FIG. 6 shows a cross-section schematic of the testapparatus 60. In this test apparatus 60, Teflon hold downs 64 define aninitial height. A blank sheet 62 of leadframe material (with a thicknessh of about 0.10 inch) has an insulation layer 67 above it. Below the diepad blank 62 are two pieces of metal tubing 68 (about 0.196 inch indiameter) separated by a length l of about 0.487 inch (see FIG. 6).These metal tubes 68 confine the solder 36 and promote solidificationfrom the sides toward the center. A 63/37 solder was used for theexperiments. An aluminum hot plate 70 was used to simulate the heatedheat sink member cooling.

A Moire experiment provided a surface map of the resulting die pad blank62, as shown in FIG. 7, which shows the actual surface curvature from anexperiment using the test apparatus 60 of FIG. 6. In this experiment,the surface curvature of the die pad blank 62 spanning about 0.5 inchhad a maximum bow of about 3.4 mils (see FIG. 7). Inputting thesefigures into the beam moment equations yields a stress at the interface42 of the solder 36 and the die pad blank 62 of about 9277 psi.

The results of another experiment using a larger die pad blank (length lof about one inch) are shown in FIG. 8. In this Moire experiment, a testapparatus 60 like that of FIG. 6 was used, but with a circular shaped(from a top view) support tube 68. A bow of about 4.16 mils with adiameter of about 0.875 inch was measured, as illustrated in FIG. 8.Thus, these experiments confirmed that negative pressure generated bytrapped solidifying solder 36 at or near the solder/die pad interface 42may be great enough to cause a delamination void 56 between the die 26and die pad 28 of a die package 22.

In a method of the present invention for reducing the likelihood that adie pad 28 will be delaminated from a die 26 in an integrated circuitdie package 22 during an attachment of the die pad 28 to a heat sinkmember 30 (or members), a structure or structure design may be evaluated(using the above analysis or some other currently known or laterdeveloped analysis methodology) to determine whether a volume of lastsolidification 58 for solidifying solder 36 is being trapped at or near(proximate to) an interface 42 of the solder 36 and the die pad 28. Ifthe last solidification volume 58 is at or near the interface 42 of thesolder 36 and the die pad 28, the design of the structure 40 or somepart of the structure 40 (e.g., redesigning a heat sink member 30) maybe modified so that the last solidification volume 58 will be (always ormost of the time) shifted farther away from the interface 42 of thesolder and die pad and/or shifted farther away from being atconcentrated central location. Based on the many factors that maycontribute to delamination (discussed above), there are many ways orcombinations of ways that a structure design may be modified to reducethe likelihood of delamination during solder solidification. Some ofthese ways of modifying the structure design will be described next. Anyof the modification steps discussed below may be combined in anycombination for a method of the present invention.

Upon evaluation of many sample structures, a trend was revealed that thesolder thickness is an important factor regarding the cooling pattern ofthe solidifying solder 36. For a typical structure design (e.g., FIG.2), structures 40 with a minimum solder thickness between the die pad 28and the heat sink member 30 of greater than about 4 mils were morelikely to have a trapped last solidification volume 58 of solidifyingsolder 36 at a central region and at or near the interface 42 of thesolder 36 and die pad 28. When the solder 36 is thinner (e.g., less thanabout 4 mils), it tends to solidify more uniformly across the die pad 28and is less likely to have a concentrated region of trapped solidifyingsolder 36. Therefore, one way to modify the structure design is todemand higher tolerances for the placement or formation of the heat sinkmember 30 in or on the PCB 34. It should be noted that the term “heatsink member” may be used herein to refer to one continuous piece ormultiple distinct parts that act together as a heat sink.

Another way to modify the structure design is to make the die pad 28thicker. However, the current trend is moving toward thinner die pads28. Other ways of modifying the structure design include changing thematerial choices for the die mount compound 38 (e.g., stronger bond,higher melting temperature), the solder 36 (e.g., particulates includedtherein that do not melt at reflow temperatures, lower melting pointsolder), and/or die pad 28 (or leadframe), for example. Yet anothersolution (not shown) may be a layer added to the die pad 28 prior to thesoldering to increase the strength of the die pad 28, for example.

A currently preferred way to modify the design structure in a method ofthe present invention is to modify the design of the heat sink member30. Referring again to the structure design shown in FIG. 2, one of theproblem-causing solder solidification patterns is illustrated in FIGS. 9and 10, which are representative of the actual patterns observed inFIGS. 3A-4 (discussed above). FIG. 9 is a simplified cross-section viewof the structure 40 of FIG. 2 illustrating a last solidification volumeevidenced by one or more voids 58. Note that even if air pockets 58 arenot formed at a last solidifying volume of solder 36, the negativepressure exerted on the die pad 28 by the solder 36 still may be greatenough to cause delamination. FIG. 10 is a top view of the solidifiedsolder 36 on the rivet 30 with the die pad 28 removed for illustrationpurposes (i.e., as viewed along line 10-10 of FIG. 9). FIG. 10 shows atop view of the last solidification volume region 58.

In a preferred method, the design of the heat sink member 30 is modifiedso that the last solidification volume is shifted farther away from theinterface 42 of the solder 36 and the die pad 28 than before themodification. One illustrative way to do this for a heat sink rivet 30,for example, is to have a slot 80 formed in the rivet 30. Preferably, atleast part of the slot 80 is centrally located (with respect to a diepad plane) in a top of the heat sink rivet 30. FIGS. 11-13 illustrate amodified heat sink rivet 30 in a structure 40 that shifts the lastsolidification volume 58 farther away from the interface 42 of thesolder 36 and the die pad 28. FIG. 11 is a cross-section view throughthe center of the rivet 30 to illustrate the slot 80 formed in the rivet30 and to illustrate the position of the last solidification volume 58located at least partially in the slot 80. Note that the lastsolidification volume 58 in FIG. 11 is farther away from the interface42 of the solder 36 and die pad 28 than in FIG. 9. FIG. 12 is a top viewof the solidified solder 36 on the rivet 30 with the die pad 28 removedfor illustration purposes (i.e., as viewed along line 12-12 of FIG. 11).FIG. 12 shows a top view of the last solidification volume region 58buried in the solder 36 (in hidden lines) and a top view of the slot 80(in hidden lines). FIG. 13 is a top cross-section view through the rivet30 and through the last solidification volume 58 as taken along line13-13 of FIG. 11. Using the modified heat sink rivet design shown inFIGS. 11-13, it is likely that during the solder solidification process,solder 36 will solidify along the interface 42 of the solder 36 and thedie pad 28 prior to solidifying within the slot 80. When the solder 36solidifies along the interface 42, it forms a bridge of solder 36 andeffectively thickens and strengthens the die pad 28. Hence, even thougha trapped last solidification volume 58 is formed in a central portion(relative to the die pad plane) of the structure 40, it is less likelyto displace or bow the die pad 28 because it is farther from theinterface 42 and because the bridge of solder 36 there above (on theinterface 42) tends to strengthen or support the die pad 28.

Although the slot 80 in FIGS. 11-13 extends only partly across the topof the rivet 30, in other embodiments (not shown) the slot 80 may havedifferent shapes and different lengths. Also, there may be numerousslots formed in the top of the rivet 30, which may or may not intersectwith each other. It is preferred that the slot 80 is deep enough toallow the bridge of solder 36 to form (as discussed above). The actualdepth and the length of the slot 80 for a particular structure may bedetermined with some experimentation or by simulation (e.g., computermodeling). Thus, this modification method focuses on moving the lastsolidification volume 58 vertically away from the interface 42 of thesolder 36 and the die pad 28.

Another preferred way of modifying the heat sink member 30 may focus onmoving the last solidification volume 58 away from the central portion(along the die pad plane), for example. One illustrative example of suchmodification method for another embodiment of the present invention willbe described next with respect to FIGS. 14-20.

FIG. 14 is a perspective view showing a part of structure 40 cut-awayfrom a PCB 34 (which likely has other devices attached thereto). FIG. 14is provided to provide a context reference for FIGS. 15-20. In FIG. 14,a die 26 is attached to a die pad 28, which is attached to a PCB 34 anda heat sink member 30 formed in the PCB 34. For purposes ofsimplification and illustration, the remainder of the die package 22(e.g., leads, mold compound, wires, etc.) is not shown in FIG. 14.

FIG. 15 is a perspective view of the PCB 34 without the die package 22and solder 36 to illustrate the arrangement of the heat sink member 30,which includes a set of nine metal vias 84 formed in the PCB 34. Notethat one of the metal vias 84 of this design is centrally located withthe remaining eight metal vias 84 surrounding it. It has been found thatduring the cooling of the solder 36 after a reflow process (whileattaching the die pad 28 to the heat sink member 30 with solder 36 andwhile attaching the leads of the package 22 to the PCB 34), a trappedlast solidification volume 58 for the solidifying solder 36 is centrallylocated. This is illustrated in FIGS. 16 and 17. FIG. 16 is a top viewof the solder 36 on the heat sink member 30 (design of FIG. 15) and thePCB 34 as taken along line A-A in FIG. 14. FIG. 17 is cross-section viewof the structure 40 of FIGS. 15 and 16 as taken along line B-B in FIG.14. As shown in FIGS. 16 and 17, for the heat sink design of FIG. 15,the last solidification volume 58 is likely to be centrally located andat or near the interface 42 of the solder 36 and the die pad 28. Asdiscussed above, this is not a preferred location for the lastsolidification volume 58 of the solidifying solder 36. During cooling,the centrally located metal via 84 tends to cool slower than the outermetal vias 84, which can cause the last solidification volume 58 to beabove the centrally located via 84, as illustrated in FIGS. 16 and 17.

In a method of the present invention, the design of the heat sink member30 in FIG. 15 may be modified to shift the last solidification volume 58away from a central location of the structure 40 along the die padplane. One way to do this is illustrated in FIGS. 18-20. FIG. 18 is aperspective view of the PCB 34 with the metal vias 84 as the heat sinkmember 30, which is similar to that of FIG. 15. In the PCB 34 of FIG.18, the heat sink design of FIG. 15 has been modified to remove thecentrally located metal via 84. As a result, the solder 36 is morelikely to solidify in the center during a reflow process before itsolidifies over the metal vias 84. This is illustrated in FIGS. 19 and20. FIG. 19 is a top view of the solder 36 on the heat sink member 30(design of FIG. 18) and the PCB 34 as taken along line A-A in FIG. 14.FIG. 20 is cross-section view of the structure 40 of FIGS. 18 and 19 astaken along line B-B in FIG. 14. As shown in FIGS. 19 and 20, for theheat sink design of FIG. 18, the solder solidification is likely tooccur at the central portion (with respect to the die pad 28) and on theouter edges. The last solidification volume(s) 58 are likely to be moreevenly distributed over all the metal vias 84, as shown in FIG. 19,rather than being concentrated a one central location (see e.g., FIGS.16 and 17). Even though there may still be trapped solidifying solderregions or volumes in the modified design of FIG. 18, as shown in FIGS.19 and 20, these volumes are likely to be smaller and more evenlydistributed towards the outer edges of the die pad 28. Hence, the stressconcentration at any one position on the die pad 28 should be decreasedand the effective arm of the bending moment is significantly reduced toreduce the likelihood of deflecting the die pad 28.

An advantage of modifying the heat sink member 30 is that themodification may make the thickness of the solder irrelevant or lessrelevant. In other words, some heat sink member designs (see e.g.,examples described above) may work well with a wider range of solderthicknesses than prior designs.

With the benefit of this disclosure, one of ordinary skill in the artwill likely realize many other particular ways to modify a particularheat sink design in accordance with a method of the present invention.There are many currently know heat sink designs for a structure andthere will likely be many more future developed heat sink designs forextracting heat from a die pad of die package. In any case, a method ofthe present invention may be used to evaluate and, if needed, modify adesign of the heat sink member(s) and/or the structure to reduce thelikelihood that a die pad will be delaminated from a die in anintegrated circuit die package during the attachment of the heat sinkmember(s) using solder (e.g., during a solder reflow process).

Although embodiments of the present invention and at least some of itsadvantages have been described in detail, it should be understood thatvarious changes, substitutions, and alterations can be made hereinwithout departing from the spirit and scope of the invention as definedby the appended claims. Moreover, the scope of the present applicationis not intended to be limited to the particular embodiments of theprocess, machine, manufacture, composition of matter, means, methods,and steps described in the specification. As one of ordinary skill inthe art will readily appreciate from the disclosure of the presentinvention, processes, machines, manufacture, compositions of matter,means, methods, or steps, presently existing or later to be developed,that perform substantially the same function or achieve substantiallythe same result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1-8. (canceled)
 9. A method of reducing a likelihood that a die pad willbe delaminated from a die in an integrated circuit die package for astructure design during an attachment of a heat sink member to the diepad using solder, the method comprising: evaluating a sample structureof the structure design to determine whether a volume of lastsolidification for the solder is at or near an interface of the solderand the die pad and is centrally located with respect to the die pad;and if the last solidification volume is at or near the interface of thesolder and the die pad, modifying the structure design so that the lastsolidification volume is located farther from the interface of thesolder and the die pad than before the modifying and/or is locatedfarther from a central location of the die pad in a direction along aplane of the die pad than before the modifying.
 10. The method of claim9, wherein the heat sink member is a metal rivet attached to a printedcircuit board.
 11. The method of claim 9, wherein a minimum thickness ofthe solder is greater than about 4 mils between the die pad and the heatsink member for the sample structure evaluated.
 12. The method of claim9, wherein a minimum thickness of the solder is greater than about 5.5mils between the die pad and the heat sink member for the samplestructure evaluated.
 13. The method of claim 9, wherein a minimumthickness of the solder is greater than about 7 mils between the die padand the heat sink member for the sample structure evaluated.
 14. Themethod of claim 9, wherein the modifying of the structure designcomprises requiring the heat sink member or the die pad to have a slotformed therein, wherein at least part of the slot is at a centrallocation, and wherein the top of the heat sink member is closer to thedie pad than a remainder of the heat sink member when the heat sinkmember is attached to the die pad.
 15. The method of claim 9, whereinthe heat sink member comprises a plurality of metal vias formed in aprinted circuit board, and wherein the modifying of the structure designcomprises removing one or more centrally located metal vias of theplurality of metal vias from the structure design. 16-21. (canceled)